The shadow of a single tree can offer a welcome relief from the warm summer sun. But when this tree is part of a small forest, it creates a deep cooling effect. According to a study published today in the Proceedings of the National Academy of Sciences, trees play an important role in maintaining the freshness of our towns and villages.
According to the study, good tree cover can bring down daytime temperatures from summer up to 10 degrees Fahrenheit. And the effect is noticeable from one neighborhood to another, even at the scale of a block away.
"We knew that the cities were warmer than the surrounding countryside, but we found that temperatures vary just as much inside the cities.Maintaining the most comfortable temperatures on hot summer days can do all the difference for those of us who live and work there, "says Monica Turner, professor at the Faculty of Integrative Biology at the University of Wisconsin-Madison and co-author of the study.
As climatic changes make the episodes of extreme heat more and more frequent every summer, planners strive to prepare themselves. Heat waves increase energy demands and costs and can have serious consequences for human health. The authors of the study argue that one of the potentially powerful tools is the organisms that existed long before human civilizations could appreciate their benefits in terms of leaves. And these trees can be the secret so that the places where we live are habitable.
According to Turner, impervious surfaces, such as roads, sidewalks and buildings, absorb the heat of the sun during the day and release it slowly at night. Trees, on the other hand, not only shade these surfaces from the sun's rays, they sweat or release water into the air through their leaves, which cools things down.
The study found that to get the maximum benefit from this cooling service, the forest cover was to exceed 40%. In other words, a single block aerial photo of the city should be covered halfway through a network of green leaves and branches.
According to Carly Ziter, lead author of the article, studies of this type have generally focused on what is known as the "urban heat island" effect. These studies often use satellites to measure the surface temperature of the ground or to measure the air temperature inside and outside the city. Studies have shown that developed and less vegetated cityscapes are much warmer than surrounding rural lands. But this study, said Ziter, allowed researchers to analyze temperatures on a much finer scale, up to the spaces "where we live our daily life in the city".
She adds that the "heat island" effect is more like what some scientists have called an "archipelago of heat", with smaller heat islands in a city interspersed with cooler shadow areas.
To obtain data at this local scale, Ziter and his collaborators had to be creative with their sampling methods.
Satellite measurements of soil surface temperature do not really provide data on air temperature, says Ziter. They "do not bring you closer to what people really feel".
But deploying enough air temperature sensors in the city to get the precise resolution they wanted was far too expensive. Ongoing research at the University of Wisconsin-Madison has fixed temperature sensors at 150 utility poles in the city and surrounding countryside, but these sensors were often a mile or more away, far too far to provide real-time data on temperatures in individual boulevards.
In the end, Ziter opted for a reduced solution to his sampling problem. All she needed was a sensor and two wheels.
In the summer of 2016, it was not uncommon to see Ziter bike around the city of Madison with a small weather station attached to the back of his bike. In all, she traveled ten different transects in the city several times at different times of the day. The sensor on his bike indicated his location and raised the temperature of the air every second during his ride, thus generating real – time data every five meters.
In total, she estimates to have traveled 400 to 500 km and that she was "in very good shape" at the end of the study. She also collected a huge amount of data showing how trees play an important role in moderating heat in cities.
"The forest cover can do more than compensate for the effects of impervious surfaces," says Ziter. During the day, "an equivalent amount of vegetation cover can cool the air more than the pavement warms it".
The data shows that 40 percent of forest cover is the threshold required to trigger the significant cooling effects that trees have to offer. The most important cooling occurs once this threshold is crossed, that it is superior to a district of the city.
"It's not enough to plant trees, we really have to think about how many plants we plant and where we plant them," she says. "We are not saying that planting a tree does nothing, but you will have a bigger effect if you plant a tree and your neighbor plants a tree and his neighbor plants a tree."
To achieve the best value for money, urban planners should focus on the shattering areas near the threshold of forty percent above that threshold by planting trees. But, she warns, it should be in places where people are active and live, not just in parks. She adds that "we also do not want to leave the low-lying areas of our city" because they tend to be low-income neighborhoods and marginalized communities. "We want to avoid advocating for policies simply because the rich enrich them," she said.
His findings, says Ziter, underscore the importance of urban landscaping to make neighborhoods liveable in the future. It is also a call to stakeholders to work together on their trees. It is not uncommon for "different people to be responsible for different spaces," she says. For example, the city may be responsible for planting trees along its streets, while the parks department oversees the plantings in the parks and the owners decide their own private lands.
It is important that we start putting ourselves on the same page, says Ziter, because "the trees we are planting now or the areas we are paving will determine the temperatures of our cities in the next century".
From the Times Square to the Times Square, Wisconsin researchers say that if we want the places we live to be more comfortable and resilient in future climate scenarios, someone will have to speak for the trees.
Trees with grassy areas reduce heat in the summer
Carly D. Ziter et al., "Scale-dependent interactions between forest cover and impervious surfaces reduce urban heat by day in summer," PNAS (2019). www.pnas.org/cgi/doi/10.1073/pnas.1817561116